Serpentine Vegetation: Past and Present

Summary:  The serpentine ecosystem was classified as "barren" by English surveyors during the 1700s as barren was their term for landscapes without ("bare of") timber.  Tree species were present, mainly oaks, but they were open-grown and not the tall, long-trunk trees found in the fireless forests of today.  The individual serpentine areas and surrounding landscapes were primarily in savanna condition due to frequent and widespread low-intensity ground fires ignited by Native Americans mainly for fire-hunting white-tailed deer.  (Savannas are dominated by grasses but, in contrast with grasslands such as prairies, savannas have an open canopy of trees with a canopy coverage of at least 5 - 10 %.)  Fire in this fire-frequent oak savanna ecosystem ended about 1730, with the demise of Native American tribes, and the ecosystem changed quickly without fire.  Savanna developed into forest unless subject to grazing by livestock and other disturbances.  By 1800, timber-size trees were available for harvest, and before 1914, all timber stands at Soldiers Delight were logged.  Beginning in the 1930s, in the absence of fire and grazing, the non-indigenous Virginia pine and greenbrier rapidly destroyed most native vegetation.  Due to fire exclusion, grazing, cutting, pine-greenbrier invasion, and excessive deer browsing, the extent of the current oak savanna at Soldiers Delight is smaller than before settlement (ca. 1750).  Today, oaks continue to expand and grow in restoration areas, and ground vegetation is dominated by little bluestem (Schizachyrium scoparium) and other grasses, with a variety of wildflowers, sedges, and rushes.

Serpentine vegetation has been, and continues to be, a product of climate change.  Maryland climate toward the end of the Pleistocene, 20,000 to 17,000 years before the present (BP), was very cold and dry as the Laurentide Ice Sheet was nearby in Pennsylvania.  The main forest type in the piedmont was an open conifer forest dominated by spruce, fir, and white pine, named the cool temperate conifer forest.  Between 16,000 and 8,000 BP, plant distributions and abundances changed rapidly across North America (Williams et al. 2004).  Cold-tolerant trees migrated northward and were replaced by deciduous ones, especially oaks.  During the warm and dry periods of the mid-Holocene Hypsithermal Interval, 8,000 BP to 4,000 BP, the temperate deciduous forest, dominated by oaks, was fully developed in the mid-Atlantic region.  Like the temperate conifer forest, this deciduous forest had an open canopy and, therefore, more herbaceous cover than deciduous forest of today.  The serpentine oak savanna ecosystem could be thousands of years old, based on knowledge about the development and expansion of prairie and savanna in the Midwest.  Warm and dry periods of the Hypsithermal Interval resulted in major expansion and migration of prairie and savanna, including an eastward migration from the Midwest augmenting the regional flora already in place.  Those plants which tolerated magnesium-rich calcium-poor soils would have been able to colonize serpentine, notably blackjack oak, post oak, little bluestem, big bluestem, and Indian grass.  As in the Midwest, Amerindian landscape fires would have aided the migration and expansion of savanna plant populations especially during periodic wet cycles.  Their maintenance would also have been assisted by periodic megadroughts after the Hypsithermal, the most recent ending just before English settlement in the 1700s.  

Discussion:

Vegetation 1700 - Present

            In the 1700s, the indigenous serpentine ecosystem was classified as "barren" by English surveyors (details in Landscape History section).  Barren was the term used by the English for landscapes without ("bare of") timber.  Tree species were present, mainly oaks, but they were not the tall, long-trunk trees found in the fireless forests of today.  The individual serpentine areas and surrounding landscapes were primarily in savanna condition due to frequent and widespread low-intensity ground fires ignited by Native Americans.  Savannas are dominated by grasses but, in contrast with grasslands such as prairies, savannas have an open canopy of trees with a canopy coverage of at least 5 - 10 % (Anderson et al. 1999).  Grassland and open forest were present, but minor in comparison to the coverage of savanna.  The main savanna tree species at Soldiers Delight were blackjack oak (Quercus marilandica), post oak (Q. stellata), and sassafras (Sassafras albidum) (Weakley 2015 nomenclature).

            Vegetation type at Soldiers Delight and other serpentine areas is partially controlled by soil type.  The shallowest soils are only a few inches deep, well drained, and drought-prone (details in Soils and Vegetation section).  Where this soil is only 2 - 3 inches deep, only grassland vegetation was able to persist as woody plants need deeper soils and more moisture for their rooting systems.  On deeper soils, the English reported oak and sassafras "bushes", and "saplings" on even deeper soils.  And the deepest soils provided both adequate rooting volume and moisture for the development of large open-grown trees such as "gnarled stunted oaks".   

            Fire in this fire-frequent oak savanna ecosystem ended about 1730, with the demise of Native American tribes, and the ecosystem changed quickly without fire.  While the shallowest soils remained grassland, savanna developed into forest (Marye 1955).  By 1800, timber-size trees were available for harvest, and before 1914, all forests in Soldiers Delight were logged (Besley 1914).

            The current grassland community at Soldiers Delight is much larger than before settlement, due to grazing, cutting, and other activities after settlement (ca. 1750), and excessive deer browsing in recent decades (see Landscape History section).  With effective stewardship, oak and sassafras recolonization will convert much of the current grassland into savanna.  This is an important process since savannas are even more diverse than grasslands primarily because of the oaks and the numerous insects which feed upon them.  In addition, oak trees create shaded habitat for forbs which cannot tolerate or compete well in full sunlight conditions (Pavlovic et al. 2006).    

            Grassland vegetation today is primarily dominated by little bluestem (Schizachyrium scoparium), with Indian grass (Sorghastrum nutans) more dominant in some areas.  These grasses are the same plants which are dominant in prairies and savannas of the Midwest.  In spring, seasonally characteristic plants in the grasslands at Soldiers Delight are parasol sedge (Carex umbellata), round-fruited witchgrass (Dichanthelium sphaerocarpon), serpentine chickweed (probably a variety or subspecies of Cerastium velutinum), and lyreleaf rockcress (Arabidopsis lyrata).  In summer, conspicuous plants are papillose nutrush (Scleria pauciflora), Appalachian ragwort (Packera anonyma), starved witchgrass (Dichanthelium depauperatum), and grey goldenrod (Solidago nemoralis).  In fall, serpentine aster (Symphyotrichum depauperatum), Indian grass (Sorghastrum nutans), arrowfeather (Aristida purpurascens), fork-tip three-awn grass (Aristida dichotoma), and glade knotweed (Polygonum tenue) are characteristic.  The relative abundance of these, and less common plants, can be found in Tyndall (1992), and detailed botanical descriptions can be found in Weakley (2015).

            The flora of serpentine oak savanna unfortunately was not documented before Soldiers Delight was invaded by non-indigenous and highly invasive Virginia pine and greenbrier, beginning in the 1930s (see Landscape History section).  However, a remnant of oak savanna was sampled after it was uncovered in 1991 by the removal of a dense pine stand (Tyndall 2005).  Before pine clearing, plant cover was sparse under the pines, but it had tripled in just 12 years after clearing, with little bluestem becoming the dominant plant.  In addition, the oak savanna flora was somewhat different from adjacent grassland, adding additional diversity to the ecosystem, and it was still changing at the end of the study. 

Vegetation Before 1700

            Serpentine vegetation has been, and continues to be, a product of climate change.  Rrecords do not exist specifically for Soldiers Delight, but excellent information is available about pre-historical climates and vegetation types for the mid-Atlantic region, Chesapeake Bay area, and the Midwest.  That information provides insight into the development of serpentine oak savanna in the piedmont of Maryland, with the understanding that much more research is needed to fill in the blanks of current knowledge.

            Maryland climate toward the end of the Pleistocene, 20,000 to 17,000 years before the present (BP), was very cold and dry as the Laurentide Ice Sheet was nearby in Pennsylvania.    The main forest type in the piedmont was an open conifer forest dominated by spruce, fir, and white pine, named the cool temperate conifer forest (Delcourt and Delcourt 1984, Williams et al. 2000).  This conifer forest was not a closed canopy forest because of dry conditions and low CO₂ levels.  Prairie forbs were present, but low in number (Williams et al. 2004).  Oaks were essentially limited to the southeastern United States during this glacial maximum.

            Between 16,000 and 8,000 BP, plant distributions and abundances changed rapidly across North America (Williams et al. 2004).  Plant ranges expanded in both south-north and west-east directions, and centers of abundance also changed during this period.  Cold-tolerant trees migrated northward and were replaced by deciduous ones, especially oaks.  Oaks migrated rapidly between 20,000 and 14,000 BP, and may have become most abundant in Maryland between 10,000 and 9,000 BP (Williams et al. 2000).  Although common today, oaks were more abundant in the past.

            During warm and dry periods of the mid-Holocene Hypsithermal Interval, 8,000 BP to 4,000 BP, the temperate deciduous forest, dominated by oaks, was fully developed in the mid-Atlantic (Williams et al. 2000).  Like the temperate conifer forest, this deciduous forest had an open canopy and, therefore, more herbaceous cover than deciduous forest of today (Williams et al. 2004).  In the southern portion of the Chesapeake Bay area, oaks became mixed with southern pines during 5500 BP to 4800 BP, as pines migrated northward in response to warmer and wetter winters (Willard et al. 2005), adding a mixed deciduous-conifer forest to the region.  Both of these forest types remained prominent until widespread deforestation by the English beginning in the 1700s AD in Maryland.

            Between 1450 AD and 2000 AD, the Chesapeake Bay region experienced 14 wet-dry climatic cycles, and some of the dry cycles were extreme (Cronin et al. 2000).  Megadroughts, lasting for more than a decade, occurred during dry cycles between 1450 AD and 1650 AD, and some were more severe than the Dust Bowl of the 1930s.  These megadroughts also occurred during the coolest part (1450 - 1600 AD) of the Little Ice Age (1300 - 1900 AD) (Cronin et al. 2003).  The end of the 1450 - 1650 AD megadroughts nears the beginning of historical descriptions of the serpentine barrens by English surveyors and colonists (Marye 1955).  Precipitation increased in the late 1600s, and wet conditions continued until about 1930.

            A similar but more complex pattern of climate and vegetation change occurred in the Midwest, where spruce and jack pine were dominant at the peak of glaciation about 18,000 BP (Anderson 2006).  As climate warmed and dried during the early Holocene, 10,000 - 8,000 BP, conifers were replaced by grassland, oak savanna, and open oak-hickory forest.  Between 8,000 and 5,000-3500 BP, depending on the location, the Hypsithermal Interval had cycles of even warmer and drier conditions, and the landscape was subject to frequent fires ignited by Native Americans (Anderson 2006, Moore 1972, Nelson et al. 2006).  As a result, oak-hickory forest was converted to oak savanna and prairie, except in the fire shadows of steep slopes, lakes, and rivers (Anderson 2006, Thomas-Van Gundy 2020).  During wet cycles of the Hypsithermal and the wetter climate following it, frequent landscape fires prevented re-expansion of the oak-hickory forest and maintained prairie and savanna.  Prairie expanded farther eastward, after a wet pause, ending about 6200 BP, reaching into central Ohio (Mack and Boerner 2004).  Dominant grasses included two which are dominant at Soldiers Delight, Indian grass and little bluestem.  Although wet cycles occurred after prairie expansion, frequent landscape fires maintained prairies as well as oak savannas, woodlands, and open forests until European settlement in the 1800s (Nelson et al. 2006, Thomas-Van Gundy 2020).            

            The serpentine oak savanna ecosystem could be thousands of years old, based on knowledge about the development and expansion of prairie and savanna in the Midwest.  The warm and dry periods of the mid-Holocene Hypsithermal Interval, 8,000 - 4,000 BP, allowed prairie and savanna plants to migrate eastward, augmenting the regional flora already in place.  Those that tolerated magnesium-rich calcium-poor soils were able to colonize serpentine, notably blackjack oak, post oak, little bluestem, big bluestem, and Indian grass.  As in the Midwest, Amerindian landscape fires would have aided the migration and maintenance of savanna plant populations during periodic wet cycles.  Their maintenance would also have been assured by periodic megadroughts after the Hypsithermal, the most recent ending just before English settlement in the 1700s.  

References:

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Besley, F.W. 1914. Map of Baltimore County and Baltimore City showing the forest areas by commercial types. Maryland Board of Forestry, Baltimore, Maryland.

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